Methods for extracting hemicellulose from a cellulosic material

ABSTRACT

A method for treating a cellulosic material comprises extracting the cellulosic material with an extractant to selectively extract hemicellulose therein and separating the extracted hemicellulose to form a cellulosic product comprising less hemicellulose than the cellulose-containing material. The extractant comprises an ionic liquid and a non-solvent comprising acetic acid. The cellulosic product retains the cellulosic fiber morphology.

CROSS-REFERENCE TO RELATED APPLICATION

This is a non-provisional of U.S. Provisional Application 61/684,993, filed Aug. 20, 2012, the entirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the extraction of hemicellulose from cellulose containing materials. In particular, the present invention relates to processes for extracting hemicellulose from cellulose containing materials using an extractant comprising acetic acid.

BACKGROUND OF THE INVENTION

Cellulose is typically obtained from wood pulp and cotton and may be further modified to create other derivatives including cellulose ethers, cellulose esters and cellulose nitrate, among others. Cellulose derivatives have a variety of commercial uses. For example, cellulose acetate is the acetate ester of cellulose and is used for a variety of products, including textiles (e.g., linings, blouses, dresses, wedding and party attire, home furnishings, draperies, upholstery and slip covers), industrial uses (e.g., cigarette and other filters for tobacco products, and ink reservoirs for fiber tip pens, decking lumber), high absorbency products (e.g., diapers, sanitary napkins, and surgical products), thermoplastic products (e.g., film applications, plastic instruments, and tape), cosmetic and pharmaceutical (extended capsule/tablet release agents and encapsulating agent), medicinal (hypoallergenic surgical products) and others.

High purity α-cellulose is required as a starting material to make cellulose derivatives, such as cellulose acetate. Acetate-grade pulps are specialty raw materials produced in commercial pulp processes, but the cost for such pulps is high. Commercial paper grade pulps contain less than 90% α-cellulose and are potential crude cellulosic sources for making cellulose derivatives. However, paper grade pulp also contains a high amount of impurities, such as hemicellulose, rendering it incompatible with certain industrial uses, such as making acetate flake or tow.

Zhou et al. discusses the use of dimethyldioxirane (DMDO), a pulp bleaching agent, to treat birch pulp and obtain acetate-grade pulp. However, DMDO is not commercially available due to its instability. Therefore, it is not an ideal solvent for producing large quantities of high α-cellulose pulp. Zhou et al. “Acetate-grade pulp from birch,” BioResources, (2010), 5(3), 1779-1778.

Studies have been done regarding the treatment of biomass to form biofuels. Specifically, it is known that various ionic liquids can be used to dissolve cellulosic material. S. Zhu et al. in Green Chem. 2006, 8, pp. 325-327, describe the possibility of dissolving cellulose in ionic liquids and recovering it by addition of suitable precipitates such as water, ethanol, or acetone.

Others have used ionic liquids to break down the cellulosic materials to make biofuels by way of glucose. For example, U.S. Pat. Appl. 2010/0112646 discloses a process for preparing glucose from a cellulose material, in which a cellulose-comprising starting material is provided and treated with a liquid treatment medium comprising an ionic liquid and an enzyme. Similarly, U.S. Pat. Appl. 2010/0081798 discloses a process for preparing glucose from a material containing ligno-cellulose, in which the material is first treated with an ionic liquid and then subjected to enzymatic hydrolysis. U.S. Pat. Appl. 2010/0081798 describes obtaining glucose by treating a material containing lingo-cellulose with an ionic liquid and subjecting same to an enzymatic hydrolysis and fermentation. However, in order to turn cellulose containing materials into glucose, the methods disclosed in these references result in breaking down the cellulose molecules, making them unsuitable for use as starting materials to make cellulose derivatives.

U.S. Pat. No. 7,828,936 describes a method for dissolving cellulose in which the cellulose based raw material is admixed with a mixture of a dipolar aprotic intercrystalline swelling agent and an ionic liquid. This method results in the complete dissolution of the cellulose and destruction of the fiber morphology of the cellulose. Although the cellulose may be regenerated using a non-solvent, the crystallinity of the regenerated cellulose is lower than the original cellulose sample.

Therefore the need exists for methods for producing high purity α-cellulose from lower grade starting materials without destroying the fiber morphology and other characteristics of the cellulose structure. In particular, the need exists for cost effective methods for removing hemicellulose from cellulosic materials to recover high purity α-cellulose that can be converted to other cellulose derivatives.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a process for treating a cellulosic material, comprising: extracting hemicellulose from the cellulosic material with an extractant, wherein the extractant comprises an ionic liquid and a non-solvent comprising acetic acid; and separating the extracted hemicellulose from the cellulosic material to form a cellulosic product comprising less hemicellulose than the cellulosic material. The extractant may comprise from 10 to 40 wt. % acetic acid and from 60 to 90 wt. % ionic liquid. The cellulosic product may comprise at least 10% less hemicellulose than the cellulosic material. In some embodiments, less than 15% of the cellulose in the cellulosic material is extracted. The cellulosic product may retain a cellulosic fiber morphology. The ionic liquid may be selected from the group consisting of imidazolium salts, pyridinium salts, ammonium salts, and phosphonium salts. In some embodiments, the ionic liquid is 1-ethyl-3-methyl imidazolium acetate. The non-solvent may further comprise at least 5 wt. % water. The non-solvent may be free of dimethyl sulfoxide. The process may further comprise treating the cellulosic material with an enzyme, e.g., hemicellulose. The extracting step may be conducted at a temperature from 30° C. to 150° C. or from 90° C. to 120° C. The extracting step may occur for between 5 minutes and 180 minutes. The process may further comprise washing the cellulosic product with at least one of dimethylformamide, N-methyl pyrrolidone, methanol, ethanol, isopropanol, dimethyl carbonate, acetone and/or water. The process may further comprise repeating the extraction step to remove additional hemicellulose. The cellulosic product may have an absorbance of less than 2.0 at 277 nm, of less than 1.8 or of less than 1.5. In some embodiments, the cellulosic material comprises wood pulp, e.g., paper grade wood pulp.

In a second embodiment, the present invention is directed to a cellulosic material formed by extracting hemicellulose from a cellulosic material with an extractant, wherein the extractant comprises an ionic liquid and a non-solvent comprising acetic acid; and separating the extracted hemicellulose from the cellulosic material to form a cellulosic product comprising less hemicellulose than the cellulosic material.

In a third embodiment, the present invention is directed to a process for producing high purity α-cellulose grade pulp, comprising: extracting hemicellulose from a wood pulp by treating the wood pulp with an extractant comprising an ionic liquid and a non-solvent comprising acetic acid to form an intermediate wood pulp that retains a cellulosic fiber morphology; treating the intermediate wood pulp with an enzyme to hydrolyze hemicellulose contained therein and forming a reduced hemicellulose wood pulp; and washing the reduced hemicellulose wood pulp with a wash solution to form the high purity α-cellulose grade pulp.

In a fourth embodiment, the present invention is directed to a high purity α-cellulose grade pulp product purified by removing hemicellulose from cellulosic material using a combination of extraction with ionic liquids and acetic acid, and enzymatic hydrolysis of hemicellulose by hemicellulose-selective enzyme, wherein the high purity α-cellulose grade pulp product comprises less than 5% hemicellulose.

DETAILED DESCRIPTION OF THE INVENTION Introduction

Methods according to the disclosure are suitable for removing impurities, such as hemicellulose, from a cellulosic material. Generally speaking, the term “cellulosic material” includes cellulose and hemicellulose. The process of the invention generally includes the step of extracting the cellulosic material with an extractant to selectively extract hemicellulose. Preferably, the extractant includes an ionic liquid and a non-solvent comprising acetic acid. The non-solvent may further comprise water. In some embodiments, the non-solvent is free of dimethyl sulfoxide (“DMSO”). Without being bound by theory, it is believed that by excluding DMSO from the extractant, downstream recovery of the extrantant is improved. This may, in part, be due to the lower boiling point of acetic acid as compared to DMSO.

The extracted hemicellulose is then separated from the cellulosic product. As a result, the cellulosic product has less hemicellulose than the starting cellulosic material, e.g., at least 10% less, at least 20% less or at least 30% less hemicellulose than the starting cellulosic material.

In one embodiment, the cellulosic material may be subjected to one or more extractions with an extractant. The extractant includes an ionic liquid that is suitable for extracting hemicellulose in the cellulosic material. The use of ionic liquid alone would dissolve both hemicellulose and α-cellulose thereby destroying the fiber morphology of the cellulosic material and increasing the degree of difficulty for separation and recovery of the cellulosic products. Once destroyed, the original fiber morphology of the cellulosic material disappears and cannot be regenerated. It has now been discovered that by using a mixture of ionic liquid and non-solvent comprising acetic acid at appropriate proportions, hemicellulose may be selectively extracted without destroying significant amounts of the cellulosic fiber morphology. Therefore, the fiber morphology of the cellulosic material is retained in the cellulosic product. Without being bound by theory, α-cellulose is generally insoluble in the ionic liquid/non-solvent extractant, while hemicellulose is soluble therein. As used herein, the term “non-solvent” refers to liquids that are capable of reducing α-cellulose solubility in ionic liquids to a point that the α-cellulose does not readily dissolve in a mixture of the non-solvent with an ionic liquid. Therefore, the non-solvent in combination with the ionic liquid selectively dissolves the hemicellulose but leaves the cellulose substantially intact. In addition, other characteristics of the cellulosic material are also retained, such as intrinsic viscosity and brightness. Thus, the resulting cellulosic product has a reduced amount of hemicellulose, but retains the fiber morphology, intrinsic viscosity and brightness of the cellulosic material. In one embodiment, less than 15% of the cellulose in the cellulosic material is extracted, e.g., less than 10%, or less than 5%, as determined by HPLC based carbohydrate analysis. In one embodiment, the cellulosic material may be subjected to repeated extraction with the extractant.

In one embodiment, the cellulosic material may also be treated with an enzyme to hydrolyze the hemicellulose to form a reduced hemicellulose product. It has now been discovered that by treating cellulosic materials with both the extractant and an enzyme, a substantially hemicellulose-free cellulosic product may be recovered. For example, the cellulosic product may comprise less than 5% hemicellulose, less than 3% hemicellulose, or less than 2% hemicellulose. Surprisingly and unexpectedly, the order of extractant and enzyme treatments of the cellulosic material affects the amount of hemicellulose removed from the cellulosic material. In preferred embodiments, the cellulosic material is treated with the extractant prior to enzyme treatment (although the reverse sequence of steps is also contemplated). In one embodiment, the cellulosic material may be treated with the extractant again after the enzyme treatment, optionally followed by another enzyme treatment step. The extractant and enzyme treatment steps may be further repeated as desired to obtain the desired α-cellulose purity.

According to the invention, the non-solvent comprises acetic acid. In other embodiments, the non-solvent comprises acetic acid and water. In some embodiments, the non-solvent consists of acetic acid or consists of acetic acid and water, i.e., excludes other non-solvents. In still other embodiments, the non-solvent comprises acetic acid and at least of water, dimethylformamide (DMF), (DMSO), methanol, ethanol, isopropanol, acetone, and mixtures thereof. In some preferred embodiments, the acetic acid is present in a greater amount, on a weight basis, than the other non-solvent component(s).

In one embodiment, solvents may be used to wash and remove the ionic liquid from the cellulose product. For example, DMF, methanol, ethanol, isopropanol, acetone, acetic acid, dimethyl carbonate or mixtures thereof may be used as a washing agent for the cellulosic material.

In another embodiment, acetone and/or water may be used as a rinsing agent to remove other solvents from the reduced hemicellulose product before drying to produce the final cellulose product.

Process for Removing Hemicellulose

The present invention is broadly applicable to the treatment of natural cellulose-containing materials, including plant and plant-derived materials. As used herein, the term “cellulose-containing material” includes, without limitation, plant derived biomass, corn stover, sugar cane bagasse and cane residues, rice and wheat straw, agricultural grasses, woodchips, and other forms of wood, bamboo, and all other materials proximately or ultimately derived from plants.

Generally, cellulosic material may be derived from lignin-containing materials, where lignin has been removed therefrom. In cellulosic materials, hemicellulose is linked to cellulose by hydrogen bonds. Overall, the cellulose material has a linear shape of fiber morphology, which is surrounded by hemicellulose via hydrogen bonds. These bonds between cellulose and hemicellulose may become weakened by treating the cellulosic material with an extractant to selectively dissolve the hemicellulose while maintaining the fiber morphology of the cellulose material.

The cellulosic material may be provided as pellets, sheets or chips. Exemplary sources of cellulosic material include, but are not limited to, rice straw, hard wood, hardwood pulp, soft wood, softwood pulp, herbs, recycled paper, waste paper, wood chips, pulp and paper wastes, waste wood, thinned wood, cornstalk, chaff, wheat straw, sugar cane stalk, bagasse, agricultural residual products, agricultural wastes, excretions of livestock, or mixtures thereof. In an embodiment of the invention, the cellulosic material is a paper grade pulp provided in forms such as, but not limited to, rolls, sheets, or bales. Preferably, the paper grade pulp comprises at least 70 wt. % α-cellulose, e.g., at least 80 wt. % α-cellulose or at least 85 wt. % α-cellulose. Paper grade pulp typically also comprises at least 5 wt. % hemicellulose, at least 10 wt. % hemicellulose or at least 15 wt. % hemicellulose. In another embodiment, the cellulosic material may be other α-cellulose containing pulps, such as viscose grade pulp, rayon grade pulp, semi-bleached pulp, unbleached pulp, bleach pulp, Kraft pulp, absorbent pulp, dissolving, or fluff. These cellulosic materials may be treated using the inventive method to produce high purity α-cellulose products.

Cellulose is a straight chain polymer and is derived from D-glucose units, which condense through β-1,4-glycosidic bonds. This linkage motif contrasts with that for α-1,4-glycosidic bonds present in starch, glycogen, and other carbohydrates. Unlike starch, there is no coiling or branching in cellulose and cellulose adopts an extended and rather stiff rod-like confirmation, which is aided by the equatorial confirmation of the glucose residues. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighboring chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength, which then overlay to form the marcostructure of a cellulose fiber. In an embodiment of the invention, the cellulosic product retains the fiber structure after the extraction of hemicellulose.

Hemicellulose is a polysaccharide that is typically present along with cellulose in almost all plant cell walls. Hemicellulose can be any one of xylan, glucuronoxylan, arabinoxylan, glucomannan, galactomannan, and xyloglucan. These polysaccharides contain many different sugar monomers and can be hydrolyzed.

The process of the present invention is particularly beneficial in that it has shown to be effective for use with paper grade wood pulp that is derived from softwoods and hardwoods. The method of the present invention provides a potential technique for upgrading paper grade pulp produced from softwood species, which are generally more abundant, and faster growing, than most hardwood species.

Softwood is a generic term typically used in reference to wood from conifers (i.e., needle-bearing trees from the order Pinales). Softwood-producing trees include pine, spruce, cedar, fir, larch, douglas-fir, hemlock, cypress, redwood and yew. Conversely, the term hardwood is typically used in reference to wood from broad-leaved or angiosperm trees. The terms “softwood” and “hardwood” do not necessarily describe the actual hardness of the wood. While, on average, hardwood is of higher density and hardness than softwood, there is considerable variation in actual wood hardness in both groups, and some softwood trees can actually produce wood that is harder than wood from hardwood trees. One feature separating hardwoods from softwoods is the presence of pores, or vessels, in hardwood trees, which are absent in softwood trees. On a microscopic level, softwood contains two types of cells, longitudinal wood fibers (or tracheids) and transverse ray cells. In softwood, water transport within the tree is via the tracheids rather than the pores of hardwoods.

Extractant

The extractant for extracting the cellulosic material may be any extractant capable of dissolving at least 50% of the hemicellulose, preferably at least 75% or at least 90% of the hemicellulose, in the cellulosic material. The extractant should not over-degrade the cellulose. For example, in one embodiment, the extractant dissolves less than 15% of the α-cellulose in the cellulosic material, e.g., less than 10%, or less than 5%.

In accordance to the present invention, the extractant comprises two or more components. According to the invention, the extractant comprises at least an ionic liquid and a non-solvent comprising acetic acid. In one embodiment, the non-solvent further comprises water. The ionic liquid is preferably capable of penetrating the cellulosic material. As discussed above, the ionic liquid is capable of dissolving both α-cellulose and hemicellulose. The non-solvent as used in accordance with the present invention reduces α-cellulose solubility in ionic liquids to a point that the α-cellulose does not readily dissolve in a mixture of the non-solvent with an ionic liquid. Preferably, the α-cellulose is insoluble in the non-solvent. The extractant in accordance with the present invention, therefore, has the property of selectively dissolving the hemicellulose that is in the cellulosic material.

Without being bound by theory, the insolubility of the α-cellulose in the non-solvent and the extraction agent maintains the cellulose fiber morphology, while the extractant penetrates the cellulosic material and dissolves and extracts the hemicellulose from the cellulosic material. Depending on the specific non-solvent used in the extractant, the weight percentage of the ionic liquid and the non-solvent in the extractant may vary widely. The extractant may comprise of an ionic liquid and one or more non-solvents. Depending on the non-solvent used in the extractant, the weight percentage of the ionic liquid may also vary.

In one embodiment, the extractant may comprise at least 0.1 wt. % ionic liquid, e.g., at least 1 wt. % or at least 2 wt. %. In terms of upper limits, the extractant may comprise at most 95 wt. % ionic liquid, e.g., at most 90 wt. %, or at most 85 wt. %. In terms of ranges, the extractant may comprise from 0.1 wt. % to 95 wt. % ionic liquid, e.g., from 1 wt. % to 90 wt. %, or from 2 wt. % to 85 wt. %. The extractant may comprise at least 5 wt. % acetic acid, e.g., at least 10 wt. %, at least 15 wt. %, or at least 20 wt. %. In terms of upper limits, the extractant optionally comprises at most 40 wt. % acetic acid, at most 35 wt. %, or at most 30 wt. %. In terms of ranges, the extractant optionally comprises from 5 wt. % to 40 wt. % acetic acid, e.g., from 10 wt. % to 93 wt. %, or from 15 wt. % to 30 wt. %.

In one embodiment, the non-solvent further comprises water. The water may be present from 0.1 to 99 wt. %, e.g., from 1 to 80 wt. % or from 10 to 50 wt. %. Without being bound by theory, it is postulated that the decrease in viscosity in the extractant by using the second non-solvent beneficially increases the amount of hemicellulose extracted from the cellulosic material.

Ionic Liquid

Ionic liquids are organic salts with low melting points (up to 100° C.), many of which are consequently liquid at room temperature. Specific features that make ionic liquids suitable for use in the present invention are their lack of vapor pressure, their ability to dissolve a wide range of organic compounds and the versatility of their chemical and physical properties. In addition, ionic liquids are non-flammable, which makes them suitable for use in the industrial applications.

It has been found that, in addition to these beneficial properties, when contacted with cellulosic materials, including plant matter and plant matter derivatives, the ionic liquids are capable of dissolving the hemicellulose and cellulose contained therein. In addition, with the appropriate choice of treatment conditions (for example, time of contact, temperature, and non-solvent composition), ionic liquids penetrate the structure of the cellulose-containing material to break down the material and extract organic species therein. α-Cellulosic components remaining in the cellulosic material are preserved and the fiber morphology is retained.

The ionic liquids refer to liquids consisting of ions only. Ionic liquids existing in a liquid phase at room temperature are called room temperature ionic liquids. Generally, ionic liquids are formed of large-sized cations and a smaller-sized anion. Cations of the ionic liquids may contain nitrogen, phosphorous, sulfur, or carbon. Because of the disparity in size between the cation and anion, the lattice energy of the compound is decreased resulting in a less crystalline structure with a low melting point.

Exemplary ionic liquids include the compounds expressed by the formula [A]⁺[B]⁻.

In one embodiment, the ionic liquid is selected from the group consisting of substituted or unsubstitued imidazolium salts, pyridinium salts, ammonium salts, triazolium salts, pyrazolium salt, pyrrolidinium salt, piperidium salt, and phosphonium salts. In preferred embodiments, [A]⁺ is selected from the group consisting of

wherein R, R¹, R², R³ and R⁴ are each independently selected from the group consisting of hydrogen, C₁-C₁₅ alkyls, and C₂-C₂₀ alkenes, and the alkyl or alkene may be substituted by a substituent selected from the group consisting of sulfone, sulfoxide, thioester, ether, amide, hydroxyl and amine.

[B]⁻ is preferably selected from the group consisting of Cl⁻, Br⁻, I⁻, OH⁻, NO₃ ⁻SO₄ ²—, CF₃CO₂ ⁻, CF₃SO₃ ⁻, BF₄ ⁻, PF₆ ⁻, CH₃COO⁻, (CF₄SO₂)₂N⁻, and AlCl₄ ⁻.

Examples of the compounds may include 1-butyl-3-methyl imidazolium tetrachloroaluminate, 1-ethyl-3-methyl imidazolium tetrachloroaluminate, 1-ethyl-3-methyl imidalzolium hydrogensulfate, 1-butyl-3-methyl imidazolium hydrogensulfate, methylimidazolium chloride, 1-ethyl-3-methyl imidazolium acetate, 1-butyl-3-methyl imidazolium acetate, tris-2(hydroxyl ethyl)methylammonium methylsulfate, 1-ethyl-3-methyl imidazolium ethylsulfate, 1-ethyl-3-methyl imidazolium methanesulfonate, methyl-tri-n-butylammonium methylsulfate, 1-butyl-3-methyl imidazolium chloride, 1-ethyl-3-methyl imidasolium chloride, 1-ethyl-3-methyl imidazolium thiocyanate, 1-butyl-3-methyl imidazolium thiocyanate, 1-aryl-3-methyl imidazolium chloride, and mixtures or complexes thereof, but the disclosed concept of utilizing ionic liquids is not limited to the disclosed species.

The ionic liquid may be commercially available, and may include Basionic™ AC 01, Basionic™ AC 09, Basionic™ AC 25, Basionic™ AC 28, Basionic™ AC 75, Basionic™ BC 01, Basionic™ BC 02, Basionic™ FS 01, Basionic™ LQ 01, Basionic™ ST 35, Basionic™ ST 62, Basionic™ ST 70, Basionic™ ST 80, Basionic™ VS 01, and Basionic™ VS 02, but the disclosed species is not limited thereto.

Alternatively, the compound may be 1-ethyl-3-methyl imidazolium acetate of the following structural formula (2), 1-ethyl-3-methyl imidazolium hydrogensulfate of the following structural formula (3), 1-ethyl-3-methyl imidazolium chloride of the following structural formula (4), or 1-n-butyl-3-methyl imidazolium chloride of the following structural formula (5):

Non-solvent

As stated above, non-solvents in the context of this invention include solvents that do not have the ability to readily dissolve α-cellulose. According to the invention, the non-solvent comprises acetic acid. In some embodiments, the non-solvent may further comprise at least one component selected from the group consisting of water, alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, diols and polyols such as ethanediol and propanediol, amino alcohols such as ethanolamine, diethanolamine and triethanolamine, aromatic solvents, e.g. benzene, toluene, ethylbenzene or xylenes, halogenated solvents, e.g. dichloromethane, chloroform, carbon tetrachloride, dichloroethane or chlorobenzene, aliphatic solvents, e.g. pentane, hexane, heptane, octane, ligroin, petroleum ether, cyclohexane and decalin, ethers, e.g. tetrahydrofuran, diethyl ether, methyl tert-butyl ether and diethylene glycol monomethyl ether, ketones such as acetone and methyl ethyl ketone, esters, e.g. ethyl acetate, dimethyl carbonate, dipropyl carbonate, propylene carbonate, amides, e.g., formamide, dimethylformamide (DMF), dimethylacetamide, dimethyl sulfoxide (DMSO), acetonitrile and mixtures thereof. In other embodiments, when the non-solvent comprises acetic acid, the extractant and non-solvent are free of DMSO

In one embodiment, a second non-solvent may be used in conjunction with the first non-solvent and the ionic liquid as described above. In one embodiment, the second non-solvent decreases the viscosity of the extractant. In one embodiment, the second non-solvent has a viscosity of less than 2.0 mPa·s at 25° C. In one embodiment, the second non-solvent is selected from the group consisting of formamide, dimethylformamide (DMF), dimethylacetamide, N-methylpyrrolidone, propylene carbonate, acetonitrile and mixtures thereof. It is postulated that using a low viscosity second non-solvent in the extractant, a smaller amount of ionic liquid is needed to extract the hemicellulose in the cellulosic material.

Washing Agent

In one embodiment, the cellulosic product is washed using a washing agent after the hemicellulose is extracted therefrom in the extractant. The primary purpose of the washing step is to remove residual extractant from the cellulose product as well as remove loosely bound hemicellulose contained therein. The washing agent preferably comprises a non-solvent, which cleanses the cellulose product, but may also include some low level of extractant resulting from the sequence of washing steps. In one embodiment, the washing agent is selected from the group consisting of water, alcohols, polyols, amino alcohols, aromatic solvents, halogenated solvents, aliphatic solvents, ethers, ketones, esters, formamide, dimethylformamide (DMF), dimethylacetamide, propylene carbonate, acetonitrile and mixtures thereof. In an embodiment, the washing agent is selected from the group consisting of DMF, N-methyl pyrrolidone, methanol, ethanol, isopropanol, dimethyl carbonate, propylene carbonate, acetone, water, and mixtures thereof.

Enzyme

As discussed above, in one embodiment, the cellulosic material is treated with an enzyme, preferably a hemicellulase, to break down residual hemicellulose contained in the cellulosic material. The hemicellulase includes one or more enzymes that hydrolyze hemicellulose to form simpler sugars, ultimately yielding monosaccharides (e.g., glucose, other hexoses, pentoses). Suitable hemicellulase include one or more of xyloglucanase, β-xylosidase, endoxylanase, α-L-arabinofuranosidase, α-glucuronidase, mannanase, and acetyl xylan esterase. Preferably, the enzymes include a combination of both endo-enzymes (i.e., enzymes hydrolyzing internal polysaccharide bonds to form smaller poly- and oligosaccharides) and exo-enzymes (i.e., enzymes hydrolyzing terminal and/or near-terminal polysaccharide bonds) to facilitate the rapid hydrolysis of large polysaccharide molecules. Suitable commercial hemicellulase include SHEARZYME (availab)e from Novozymes A/S, Bagsvaerd, Denmark), PULPZYME (available from Novozymes A/S, Bagsvaerd, Denmark), FRIMASE B210 (available from Puratos, Groot-Bijgaarden, Belgium), FRIMASE B218 (available from Puratos, Groot-Bijgaarden, Belgium), GRINDAMYL (available from Danisco, Copenhagen, Denmark), ECOPULP TX200A (available from AB Enzymes, Darmstadt, Germany), MULTIFECT Xylanase (available from Genencor/Danisco, Palo Alt), USA), PENTOPAN Mono BG (available from Novozymes, Bagsvaerd, Denmark), and PENTOPAN 500 BG (available from Novozymes, Bagsvaerd, Denmark).

The enzymes generally can be used in amounts that are not particularly limited. For example, hemicellulase can be used in amounts ranging from about 0.001 mg/g to about 500 mg/g (e.g., about 0.05 mg/g to about 200 mg/g, about 0.1 mg/g to about 100 mg/g, about 0.2 mg/g to about 50 mg/g, or about 0.3 mg/g to about 40 mg/g). The concentration units are milligrams of enzyme per gram of cellulosic material to be treated.

Processing Steps

In the process of the present invention, cellulose materials are treated with an extractant, which includes an ionic liquid and a non-solvent comprising acetic acid. The cellulose materials are treated by extraction wherein hemicellulose is dissolved and washed, wherein the extractant and hemicellulose is separated from cellulose materials. The extracted cellulose materials are recovered for further use. In another embodiment, the extractant is recovered and may be recycled. In another embodiment, the process may further include enzymatic digestion of hemicellulose, extraction and/or isolation of digested hemicellulose and recovery of a cellulosic product with reduced amount of hemicellulose.

One suitable treatment comprises extracting the cellulosic material with the extractant to selectively dissolve hemicellulose, and thereafter separating the dissolved hemicellulose in the liquid phase from the solid cellulosic product. The solid cellulosic product retains a cellulosic fiber morphology.

In one embodiment, the solid cellulosic product may be washed using a washing solution, which may have the property of dissolving hemicellulose. Therefore, any remaining hemicellulose in the cellulosic product may be dissolved and removed during the washing step.

In one embodiment, the cellulosic product may be subjected to repeat extraction. In one embodiment, the cellulosic product may be treated with the extractant after the initial extraction to further extract any remaining hemicellulose. In one embodiment, the cellulosic product may be treated with the extractant for a second time after the washing step. In some embodiments, the cellulosic product may be subjected to a third or fourth extraction.

In one embodiment, the cellulosic material may be subjected to enzyme treatment. In one embodiment, the cellulosic material may be treated with enzyme after the extraction step. The inventors have found that more hemicellulose is removed from the cellulosic material when the cellulosic material is first treated with the extractant. Without being bound by theory, it is believed that by treating the cellulosic material first with the extractant, enzyme could penetrate the cellulosic material more easily to hydrolyze the hemicellulose. In comparison, less hemicellulose is removed from the cellulosic material when it is first treated by enzyme and followed by extraction.

In one embodiment, after treating with extractant and enzyme, the cellulosic product may be treated with the extractant again to further remove additional hemicellulose.

Washing

In one embodiment, the cellulosic material may be washed using a washing agent to wash and remove hemicellulose and/or enzyme. In one embodiment, the cellulosic material may be washed between extraction treatments. In one embodiment, the cellulosic material may be washed between extraction treatment and enzyme treatment. In one embodiment, the washing agent may be selected from the group consisting of dimethylformamide (DMF), N-methyl pyrrolidone, dimethyl acetamide, dimethyl carbonate, propylene carbonate, water, and a mixture thereof.

The treatment of the cellulosic material may be conducted at an elevated temperature, and preferably atmospheric pressure or slightly above atmospheric pressure. Preferably the contacting is conducted at a temperature from 30° C. to 150° C., e.g., from 50° C. to 130° C., or from 90° C. to 120° C. In terms of upper limit, the treatment of the cellulosic material may be conducted at a temperature of less than 150° C., e.g., less than 130° C., or less than 125° C. In terms of lower limit, the treatment of the cellulosic material may be conducted at a temperature of greater than 30° C., e.g., greater than 50° C., or greater than 70° C. The pressure is in the range from 100 kPa to 150 kPa, preferably from 100 kPa to 120 kPa, more preferably from 100 kPa to 110 kPa.

The cellulosic material may contact the extracting agent between 5 minutes to 180 minutes, e.g., between 40 minutes to 150 minutes, or from 60 minutes to 90 minutes. In terms of lower limits, the treatment of the cellulosic material may be contacted for at least 5 minutes, e.g., at least 10 minutes or at least 20 minutes. In terms of upper limits, the treatment of the cellulosic material may be contacted for at most 180 minutes, e.g., at most 150 minutes, at most 120 minutes, or at most 60 minutes.

The extraction process may be conducted in a batch, a semi-batch or a continuous process with material flowing either co-current or counter-current in relation to one another. In a continuous process, the cellulosic material contacts the extractant in one or more extraction vessels. In one embodiment, the extractant may be heated to the desired temperature before contacting the cellulosic materials. In one embodiment, the extraction vessel(s) may be heated by any suitable means to the desired temperature.

Solid/Liquid Ratio

In the extraction step, the solid/liquid ratio may be from 0.5/100 to 1/6, and depends on extraction apparatus and set-up. In one embodiment, the solid/liquid ratio of 1.25/100 may be used to facilitate the filtration operation in a batch process. In other embodiment, solid ratio of 1/10 to 1/6 can be used in the countercurrent extraction process.

Counter-Current Extraction

The amount of extractant required has a significant impact on process economics. Counter-current extraction may achieve greater extraction efficiency while maintaining a reasonable extractant usage requirement. Counter-current extraction of solubles from pulp can be accomplished in a variety of commercial equipment such as, but not limited to agitated tanks, hydrapulpers and screw extractors. Twin-screw extractors are generally more efficient than single-screw extractors. Suitable commercial equipment to effect the separation of solid and liquid phases after extraction includes filters, centrifuges, and the like.

Good liquid/solid contact during extraction also depends on the drainage characteristics of the pulp. The particle size of pulp may be important in continuous counter-current extraction because very fine particles tend to compact and cause liquid to channel or block liquid flow completely. Extractant temperature may also have an effect on the extraction of solubles from the pulp.

Rinsing and Drying

The cellulose product may be rinsed using a rinsing agent to remove the extractant, enzyme, or dissolved hemicellulose. In one embodiment, the rinsing agent may be selected from the group consisting of water, alcohols, polyols, amino alcohols, aromatic solvents, halogenated solvents, aliphatic solvents, ethers, ketones, esters, formamide, dimethylformamide (DMF), dimethylacetamide, propylene carbonate, acetonitrile and mixtures thereof. Specifically, acetone may be use at a last rinsing agent to remove traces of water or other chemicals from the cellulose product.

Cellulose Product

In accordance to the present invention, high purity α-cellulose product is produced. In particular, these high purity α-cellulose products are high purity dissolving grade pulps with less than 5% hemicellulose. In one embodiment, the cellulosic product has an absorbance of less than 2.0 at 277 nm, e.g., less than 1.8 at 277 nm, or less than 1.5 at 277 nm. Paper grade pulp typically has an absorbance of over 4.7 at 277 nm. Purity of the α-cellulose product is indicated by a lower absorbance at a certain wavelength.

In addition to retaining the fiber morphology of the cellulosic product, the high purity α-cellulose grade pulp product also retains other characteristics such as intrinsic viscosity and brightness. The high purity α-cellulose grade pulp product may be further processed to make cellulose derivatives, such as cellulose ether, cellulose esters, cellulose nitrate, other derivatives of cellulose, or regenerated cellulose fiber, such as viscose, lyocell, rayon, etc. Preferably, the high purity α-cellulose grade pulp may be used to make cellulose acetate.

EXAMPLES

Determining Hemicellulose Content Using UV/Vis

A novel method using UV/Vis analysis method for fast determination of hemicellulose content in a sample was developed. Hemicellulose includes xylan, glucuronoxylan, arabinoxylan, glucomannan, galactomannan, and xyloglucan, while α-cellulose includes glucose. The UV/Vis analysis method is based on the fact that under the pre-hydrolysis conditions xylan degrades to form 2-furfural and glucose degrades to form 5-hydroxymethyl-2-furfural. Because the rate of degradation of the monosaccharide is different, the difference in rate of formation of furfural compounds may be used as the fundamental basis for UV/Vis method.

The UV/Vis analysis method includes a first step of pre-hydrolyzing the sample in sulfuric acid (72 wt. %) for 60±2 min at 30° C. Depending on the concentration of xylan in the sample, 10 ml or 5 ml of deionized water is added to the pre-hydrolysis solution. The solution is then measured at 270 nm to 280 nm with a SHIMADZU UV/Vis spectrometer UV-2600. The final results are adjusted by the dilution factor of the DI water dilution.

Example 1 Extraction With 1-Ethyl-3-Methylimidazoliumacetate (EMIM Ac)/Acetic Acid Extractant Binary System at 120° C.

0.8 gram of paper grade pulp sample was weighed and put into a 50 ml glass vial with Teflon face lined cap. A pre-calculated amount of EMIM Ac and Acetic Acid (total) were added into the glass vial according to the predetermined extractant weight ratio (EMIM Ac/Acetic Acid) and mixed well with the pulp samples. The sample thus contained a solid/liquid ratio (S/L) of 5%. The glass vials with pulp and extractant solution were placed into a NAPCO® Autoclave (Model 800-DSE autoclave), and the autoclave was set at 120° C. for 1 hr. After the autoclave treatment, the glass vial was left to cool to room temperature.

The content in the glass vial was transferred to a 50 ml filtration tube (0.45 um pore size filter from Grace) and centrifuged at 6000 rpm for 5 to 10 min with Thermo Fisher MR 23i Centrifuge. 1 micron or 5 micron Dutch wire cloth filter pads were used to replace the original nylon filter in the filtration tube. The pulp centrifuged on the filtration tube is collected, and the filtrate in the large tube is discarded as waste.

10 to 16 ml fresh HAc or water was added to the filter tube that contains the pulp and homogenized to form a slurry. Then it was placed into its original centrifuge filtration tube to centrifuge for additional 5-10 min at 6000 rpm. The solid and filtrate is separated via centrifuge filtration.

The color of pulp was checked to ensure that the pulp was white without any color. Additional 10 to 16 ml of water was added to wash the sample and the sample was centrifuge filtrated again. The process was repeated until the pulp appeared to be white without any color.

Following the wash, the pulp sheet from the filter pads was dispersed in 40 to 100 ml of water, and then filtered again. This water wash generally was conducted 4 times. 20 ml of acetone was added to the pulp sheet on the filter pads and then apply vacuum to dryness. The pulp sheet was left dry in chemical hood overnight.

Table 1 shows the UV/Vis absorbance of EMIM Ac as the percentage of acetic acid non-solvent changed. The absorbance of a commercially available hardwood acetate grade pulp (Comparative 1) was used as a bench mark. The absorbance of a hardwood paper grade pulp starting material (Comparative 2) was also measured and used as a comparison.

TABLE 1 UV/Vis Absorbance at 120° C. for 1 hour EMIM Ac % Acetic Acid % Absorbance @ 277 nm Comparative 2 — 4.63 50 50 4.30 60 40 4.53 70 30 3.18 74 26 1.46 75 25 1.18 75.5 24.5 1.06 76 24 1.07 76.5 23.5 1.00 77 23 1.06 77.5 22.5 1.02 78 22 1.13 Comparative 1 — 0.82

Example 2 Extraction With 1-Ethyl-3-Methylimidazoliumacetate (EMIM Ac)/Acetic Acid Extractant Binary System at 95° C.

Example 2 was prepared using the same process as Example 1, except that the extraction was performed at 95° C. The UV/Vis absorbance is shown in Table 2.

TABLE 3 UV/Vis Absorbance at 95° C. for 1 hour EMIM Ac % Acetic Acid % Absorbance @ 277 nm Comparative 2 — 4.76 74 26 1.79 75 25 1.51 75.5 24.5 1.42 76 24 1.39 76.5 23.5 1.42 77 23 1.33 77.5 22.5 1.30 78 22 1.36 Comparative 1 — 0.93

Comparative Example A Extraction With 1-Ethyl-3-Methylimidazoliumacetate (EMIM Ac)/DMSO Extractant Binary System at 95° C.

Comparative Example A was prepared using the process of Example 1 except that the non-solvent was DMSO. The DMSO was present at 97.5 wt. % of the extractant. The UV/Vis absorbance is shown below in Table 4.

Comparative Example B Extraction With 1-Ethyl-3-Methylimidazoliumacetate (EMIM Ac)/DMSO Extractant Binary System at 95° C.

Comparative Example B was prepared using the process of Example 1 except that the non-solvent was DMSO. The DMSO was present at 96.5 wt. % of the extractant. The UV/Vis absorbance is shown below in Table 4.

TABLE 4 UV/Vis Absorbance at ° C. for 1 hour EMIM Ac % DMSO % Absorbance @ 277 nm 2.5 97.5 1.227 3.5 96.5 1.125

While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. It should be understood that aspects of the invention and portions of various embodiments and various features recited above and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriately combined with other embodiments as will be appreciated by one of ordinary skill in the art. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention. 

We claim:
 1. A process for treating a cellulosic material, comprising: extracting hemicellulose from the cellulosic material with an extractant, wherein the extractant comprises an ionic liquid and a non-solvent comprising acetic acid; and separating the extracted hemicellulose from the cellulosic material to form a cellulosic product comprising less hemicellulose than the cellulosic material.
 2. The process of claim 1, wherein the extractant comprises from 10 to 40 wt. % acetic acid.
 3. The process of claim 1, wherein the extractant comprises from 60 to 90 wt. % ionic liquid.
 4. The process of claim 1, wherein the cellulosic product comprises at least 10% less hemicellulose than the cellulosic material.
 5. The process of claim 1, wherein less than 15% of the cellulose in the cellulosic material is extracted.
 6. The process of claim 1, wherein the cellulosic product retains a cellulosic fiber morphology.
 7. The process of claim 1, wherein the ionic liquid is selected from the group consisting of imidazolium salts, pyridinium salts, ammonium salts, and phosphonium salts.
 8. The process of claim 1, wherein the ionic liquid is 1-ethyl-3-methyl imidazolium acetate.
 9. The process of claim 1, wherein the non-solvent further comprises at least 5 wt. % water.
 10. The process of claim 1, wherein the non-solvent is free of dimethyl sulfoxide.
 11. The process of claim 1, further comprising treating the cellulosic material with a hemicellulase.
 12. The process of claim 1, wherein the extracting step is conducted at a temperature from 30° C. to 150° C.
 13. The process of claim 1, wherein the extracting step is conducted at a temperature from 90° C. to 120° C.
 14. The process of claim 1, wherein the extracting step is between 5 minutes to 180 minutes.
 15. The process of claim 1, further comprising washing the cellulosic product with at least one of dimethylformamide, N-methyl pyrrolidone, methanol, ethanol, isopropanol, dimethyl carbonate, acetonitrile, acetone and/or water.
 16. The process of claim 1, further comprising repeating the extraction step to remove additional hemicellulose.
 17. The process of claim 1, wherein the cellulosic product has an absorbance of less than 2.0 at 277 nm.
 18. The process of claim 1, wherein the cellulosic product has an absorbance of less than 1.8 at 277 nm.
 19. The process of claim 1, wherein the cellulosic material comprises wood pulp.
 20. The process of claim 17, wherein the wood pulp comprises paper grade wood pulp.
 21. Cellulosic material formed by the process of claim
 1. 22. A process for producing high purity α-cellulose grade pulp, comprising: extracting hemicellulose from a wood pulp by treating the wood pulp with an extractant comprising an ionic liquid and a non-solvent comprising acetic acid to form an intermediate wood pulp that retains a cellulosic fiber morphology; treating the intermediate wood pulp with an enzyme to hydrolyze hemicellulose contained therein and forming a reduced hemicellulose wood pulp; and washing the reduced hemicellulose wood pulp with a wash solution to form the high purity α-cellulose grade pulp.
 23. A high purity α-cellulose grade pulp product purified by removing hemicellulose from cellulosic material using a combination of extraction with ionic liquids and acetic acid, and enzymatic hydrolysis of hemicellulose by hemicellulose-selective enzyme, wherein the high purity α-cellulose grade pulp product comprises less than 5% hemicellulose. 